U.S. Pat. No. 7,674,854 describes a process for producing a poly vinyl acetal resin, polyvinyl butyral resin and a process for producing esterified poly vinyl alcohol resin. The resin having a high acetalization degree can be produced even in a solid catalyst system. A polyvinyl butyral resin produced by this approach can be performed at high efficiency even in a solid catalyst system.
U.S. Pat. No. 5,380,597 describes plasticized polyvinyl acetal resins based on 4-hydroxybutanal. It also describes laminar structures that include the resins, a process for making the resins and a process for making an internally plasticized acetal resin by esterification. The resin is isolated by washing and precipitation of the modified resin.
US2011/0049434 describes the modification of PVOH resin by acetalization. The modified PVOH resin composition is then capable of providing a ceramic green sheet possessing sufficient flexibility and sustains limited degree of damage even when being formed into a thin film. The method to acetalized PVOH requires the reaction mixture to be neutralized, washed and the precipitated resins are dried to provide a powder.
US2009/0093609 describes a polyvinyl acetal based resin, which has a high elastic modulus, is excellent in solubility to an alcoholic solvent and gives a highly transparent alcohol solution. The polyvinyl alcohol prepared by ATRP has 0.1 to 1.5% by mol of a 1, 2-diol structural units is first isolated and purified. The dried polymer is then dissolved and an acid is added followed by an aldehyde. The polyvinyl acetal is then isolated by neutralization and washing the resin.
US 2004/0260020 describes a method to provide a modified polyvinyl acetal resin possessing superior flexibility, adhesive property to a resin substrate under high humidities, heat resistance, thermal decomposition property, humidity toughness and low oxygen permeability. The modified polyvinyl acetal resin is obtainable by acetalizing a modified polyvinyl alcohol having ethylene in a random basis as a constituent unit of a main chain and an ethylene content of 1 to 20 mole % and a saponification degree of 80 mole % or more. An acetal modification of 40-80 mole % is required as the modified resin is water soluble and insoluble in organic solvent. Furthermore, the oxygen barrier is measured on PET with a coating thickness of 50 μm in which the coated substrate is dried at a temperature of 50 C for 6 hours and for a further 6 hours at room temperature under vacuum for oxygen permeation measurements. The modified polyvinyl acetals are dissolved in ethanol and toluene (1:1). The level of organic solvent would prevent the stable dispersion of nanoplatelets, which would limit the use of the modified acetals described in this patent. The reported oxygen transmission rates for the acetal modified polyvinyl alcohol and ethylene co vinyl alcohol copolymers are represented as oxygen transmission coefficient cc·cm/cm2·sec·cmHg when converted to cc m2-day (ASTM D3985). However, oxygen transmission rates far exceed the oxygen transmission rates of 100 cc m2/day for uncoated polyethylene terephthalate films at 12.5 μm and the transmission rates for any suitable application for barrier coatings of less than 10 cc m2/day.
However, the prior art fails to describe or suggest barrier coatings based on PVOH with an alcohol content of 40% or more which are storage stable.
The present invention described below relates a method of producing an in situ solution of a modified polyvinyl acetal in a blend of water and alcohol that when coated provides a barrier coating for a variety of applications. There are an increasing number of applications which require a co-solvent such as alcohols. A number of coating applications require the need for volatile solvents such as isopropanol, ethanol and n-propanol to allow certain coatings processes such as gravure and flexography to run at speeds >100 m/min. Typically, aqueous solutions of polyvinyl alcohol require slower speeds and high drying temperatures in coating applications to ensure the coatings dry prior to the next coating application. EVOH co-polymers offer greater alcohol tolerance than straight PVOH polymers due to the presence of ethylene on the backbone of poly vinyl alcohol. However, the drawbacks of increasing ethylene content in co-polymers while improving the alcohol tolerance is that gas barrier performance is reduced and short-term stability with high alcohol solvency is limited.